Rev. Biol. Trop., 29(2): 177-183, 1981 Comparative foragingbehav ior of Apis mellifera and Tr igona corvina (Hymenoptera: Apidae) on Baltimora recta (Compositae) by David W. Roubik * (Received for publication January 18, 1980) Abstract: In a short-term study of the foraging behavior of competing be es, European Apis mellife ra (Apinae) foraged more opportunisticalIy than Trigona corvina (Meliponinae). Experimental removal of a large portion of the available Baltimora recta (Compositae) which both bees visited caused a 44% increase in Apis within adjacent study plots of Ba/timora and only a 17% increase in Trigona. However, analysis of variance showed that the mean number of Apis �ithin study plots increased but that of Trigo na did not during three days following experimental removal of plants. Apis visited three flowers in 15.1 seconds, compared to 25.6 seconds for Trigo na. Trigona co rvina foraged in groups, and individuals occasionally attacked Apis, with no discernible effect. It is suggested that the flexible foraging behavior of A. mellifera gives it a competitive advantage compared to T. corvina when resources become more scarce. In natural cornmunities potential competitors often exhibit not only structural divergence (Selander, 1966; Karr and James, 1975; Inouye, 1978) but possess diffe ring foraging behavior as weli (MacArthur, 1958; J ohnson and Hubbell, 1975; Wolf et al., 1976; Feinsinge r and Colwell, 1978). The goal of a foraging bee is perhaps to maxirnize resource harvest relative to harvest cost. Tactics employed to this end include expenditure fo r discovery, harvest and occasionally defense of resources. During competition fo r nectar and polien, the highly social bees exhibit aggression (some Trigona) or forage nonaggressively (many Trigona, Melipona and Apis) (Johnson and Hubbell, 1974, 1975; Roubik, 1978, 1980). Competitive differences among native and introduced species are determined by fo raging tactics that evolved in separate communities. Native species may therefore display Joraging behavior that is ineffective in the presence of introduced competitors. In this study, I induced intense competition fo r a flowering plant, Baltimora recta (C ompositae) between resident Trigona (Trigona) corvina (Meliponinae) and introduced Apis mellifera (Apinae) by removing a large portion of the plants from a patch visited by both bees. The resulting change s in dispersion and foraging behavior of the bees demonstrated their differing competitive ability and fo raging tactics. * Smithsonian Tropical Research Institute, P.O. Box 2072, Balboa, Panamá. 177 178 REVISTA DE BIOLOGIA TROPICAL MATERIAL AND METHODS The observations reported here were made near Esparza, Puntaren as Province, Costa Rica (90 N) from July 13 to 16, 1978. The study site was a 2000 m2 patch of Baltimora recta on a hilltop at 208 m elevation. B. recta was the only native plant in flower at this location. Eighty-two managed colonies of j::uropean A. mellifera were 700 m from the study site. Riparian forest was present 100 m to the west and east of the study area. Eight plots measuring approximately 1.5 x 2. 5 m were selected within the study area, and bees foraging in the plots were counted' during four successive mornings from 9:00 to 11 :00 at 20-minute intervals. In addition, the time spent visiting three flower heads was measured for 50 individuals of both bee species on each day. A successful floralvisit, when the forager obtained nectar, was counted as multiple probing of florets by a bee on a flower. Brief visits involving a single probe were not counted. The elapsed foraging time for an individual bee was recorded until it had probed at least two times on each of three flower heads. On the evening of July 13 and early moming of July 14, approximately 1000 2 m of B. recta were removed from the area surrounding the study plots. The plants were cut near ground level with a machete. Within the study plots, the total flowers were estiÍnated at 6,000, ranging from 500 to 1,200 within plots. Baltimora recta is a weedy annual that flowers throughout the year (D'Arcy, 1975), and in Puntarenas, primarily between A pril and August. The plant is very abundant on cleared land grazed by cattle (G. Fuentes, pers. comm.). Since flowering occurs for several weeks, there was little likelihood that floral rewards diminished during the study. Weather conditions were relatively constant and, although the abundance and quality of nectar and pollen may have fluctuated during the experiment, the short duration of the study minimized this possibility. RESULTS Visitors of Baltimora: Foragers visiting Baltimora recta were almost exclusively Trigona corvina andApis mellifera, but T. angustula (=jaty), T capitata, T testaceicomis perilampoides, T. buyssoni, and possibly T cupira were also seen at flowers. A solitary bee, Melissodes sp. (Anthophoridae) and a cantharid beetle also visited Baltimora. Bees began to forage for nectar and pollen at 8: 15 a.m. Pollen was depleted on flowers by 10:00 a.m. since few foragers were seen carrying palien load s after this time, but foraging for nectar continued until approximately 2:30 p.m. The humidity was 95% and the temperature near 27 C each day when my observations began. Numbers of bees within study plots: The number of A. mellifera on B. recta within the study plots increased after the surrounding plants were experimentally removed (K001, ANOVA), but the number of T corvina remained stable (P<l, ANOVA, Tables 1 and 2). The average number of honeybees in combined plots on July 13 was 70, and on July 14 to 16 the number rose to near 100 (Table 1). The increase in foragers seen on these days was significant (P<O 1, range-STP test, Sokal and Rohlf, 1969). The average number of T corvina in the combined plots ranged from 50 to 61 (Table 2), but there were no significant differences between days (P>. 05, range-STP test). ROUBIK: Comparative foraging behavior of Apis and Trigona 179 TABLE 1 Apis mellifcra coun red on Baltimora recta Number 01'Bees in Combine d P10ts Time July 13 July 14 Ju1y 15 Ju1y 16 9:00 75 100 106 100 9:20 65 112 103 105 9:40 70 116 94 110 10:00 84 108 100 109 10:20 70 85 81 99 10:40 58 84 95 93 Mean* 70 101 97 103 ANOVA p<O.OOl F= 14.045 * ncarcst wholc number. Rates of floral visitation: The average duration of three successful floral visits was relatively constant for A. mellifera and increased on one day for T corvina (Table 3). A. mellifera foraged at three flowers in 14.3 to 16.1 seconds. T corvina made three visits to flowers in 23.0 to 27.9 seconds. On J uly 14, the average visitation time by T corvina increased to 27.9 seconds, greater than that observed on July 13 (P<OI, t-test). The grand means of floral visitation periods of A. mellifera and T corvina were 15.1 and 25.6 seconds/3 flowers, respectively (P<001, t test). Aggressive behavior among bees: The number of aggressive acts by T corvina increased after experimental reduction in the number of B. recta and the subsequent increase of A. mellifera within study plots. One act of intraspecific aggression was seen among T corvina on July 13, but interspecific aggression against A. mellifera was seen once on July 14, four times on July 15 and six times on July 16. Hovering T corvina alighted on Apis visiting flowers and attempted to bite with the mandibles or flew rapidly at bees in flight. The workers of A. mellifera were not aggressive. DISCUSSION Depletion of floral resources due either to competitors or floral senescence influences the distribution and activity of foraging European honeybees (Gary et al., 1977). 1 expected A. mellifera and T corvina to become more abundant in plots 180 REVISTA DE BIOLOGIA TROPICAL of B. recta after I removed most of the adjacent flowers. Further, increasing the number of bees within plots was expected to affect the amount of resource available and the foraging behavior of bees (Roubik 1978). These predictions were partially met by A. mellifera but scarcely fulfilled by T. corvina. It was not surprising that aggression by the latter species appeared to increase in frequency with increasing densities of bees, but total aggressive acts were few. TABLE 2 Trigona corvina counted on Baltimora recta Num ber of Bees in Co mbined Plots Time July 13 July 14 July 15 Jul y 16 9:00 46 58 68 62 9:20 59 62 65 58 9:40 49 73 57 48 10:00 54 71 54 49 10 :20 56 51 51 40 10 :40 46 52 56 44 Mean* 52 61 59 50 ANOVA 0.05<p<0.10 F = 2.927 * nearest whole number Was T. corvina more constant to a smaller foraging territory than honeybee foragers? Although I did not mark bees to determine their foraging area, it appears that T. corvina generally did not shift to new foraging areas after I removed flowers near the study plots. In addition to the analysis presented in Tables 2 and 3, the data show that on July 14 the number of A. mellif era increased by 44% and that of T. corvina by only 17%. Proportionally more T. corvina than A. mellifera abandoned B. recta after flowers were removed. Furthermore, the considerably larger foraging range of A. mellifera (Gary et al., 1977) would permit visitation of nearby patches of B. recta (ca. 500 m distant), while T. corvina would not be likely to reach these flowers (Michener, 1974; Hubbell and Johnson, 1977; Wille, 1976).